1. Field of Invention
This invention relates to a method for imparting decorative and printed matter onto molded products and, in particular, onto rotationally molded products.
2. Brief Statement of the Prior Art
In rotational molding, products are formed within a mold cavity by charging molding resin particles to the cavity, closing the cavity, preheating the cavity to a molding temperature and rotating or tumbling the mold, usually in two or more axes for a sufficient time to disperse the resin particles into a molten polymer film which coats the interior walls of the cavity with a coating of uniform thickness. Thereafter the mold is cooled, opened and the rotationally molded product is discharged.
An inherent disadvantage of this molding technique is that decorative or printed matter cannot readily be applied to the molded product. This disability is aggrevated by use of conventional molding resins e.g., polyolefins, such as low, medium or high density and linear polyethylene or polypropylene; ethylene-vinyl acetate copolymers, etc., which form surfaces that are not readily receptive to coatings of paints, inks and the like.
Some attempts have been made to chemically or physically treat the surfaces of rotomolded products, e.g., by flame treatment, to render the surfaces receptive to printing inks. These procedures are costly and do not result in a molded product in which the pigments or dyes of the decorative pattern or printing are incorporated into the molded skin of the product, and even the best coatings can be worn or rubbed off.
In my recent U.S. Pat. No. 4,252,762, I disclosed and claimed a method for treating the surfaces of the mold cavity by applying a decorative or printed pattern thereto using a suspension of pigments in a hydrocarbon oil. While this method incorporate decorative and printed patterns in the surfaces of rotomolded products at a high resolution, some difficulties are experienced with the process, particularly when a decorative pattern is to be applied to a surface having a complex or compound curvature, since silk screens and other stencils must be applied to essentially flat surfaces. Also stenciling is a difficult operation requiring a high degree of skill, particularly for multiple color patterns.